1. Field of the invention
The present invention, generally, relates to drive systems for belt type printers in which respective type font are attached to and carried by a moving belt and, more particularly, to a new and improved drive pulley for interconnecting such a moving belt with an electric motor of the type that is responsive to power line disturbances.
An alternating current (a.c.) motor is used customarily as the principal driver in belt type printers for many reasons, but they do have a susceptibility to follow power line frequency sometimes too well, because by doing so, they fall victim to every power line disturbance. A synchronous a.c. motor will respond to these power line disturbances with an instantaneous change in the motor lag angle, and the motor speed will change to this new lag position. When this occurs, the change in the velocity of the print belt is so fast, the print control electronics is unable to respond and bad character printing occurs.
2. Description of the Prior Art
U.S. Pat. No. 1,550,779 to Carpenter discloses a shaft coupling that is adapted especially for use in the driving connection of a washing machine, where the drum is rotated in alternating directions. It is intended to overcome the considerable momentum that is developed when a direction of rotation is reversed suddenly, which can produce shock in the transmission mechanism.
Carpenter uses a drive pin in a clearance slot in the hub to permit "slack motion", and a split spring is used to provide damping in both directions of rotation. While this structure appears to be effective in controlling shock developed in a sudden reversal of a direction of rotation, it does not concern maintaining a constant rotation in a driven shaft, such as with a structural arrangement according to the invention.
U.S. Pat. No. 1,138,779 to Scott provides a coupling that permits adjusting the hub of a shaft that is locked in place by a set screw after the new adjustment is achieved. This arrangement is not effective in maintaining a driven shaft at its given speed of rotation, as with the present invention.
A De Voe Pat. No. 1,387,043 discloses a universal joint arrangement that uses a pin to hold a driven shaft to a drive shaft, but it makes no provision for damping or for maintaining the rotational speed of a driven shaft constant during intervals of drive shaft speed variation.
A Pat. No. 2,474,347 to Collins uses a resilient material to dampen engine vibration, but is different structurally from that of the present invention. The Collins flexible drive proposes to attach a driver shaft to a driven shaft through a flexible material held to the driver shaft by four pins and this assembly is attached to a driven shaft by a pin passing through the flexible material. While this structure may be effective to dampen vibrations, it cannot maintain the driven shaft at its rotational speed while the driver shaft speed varies.
U.S. patent to Baker et al. No. 2,800,777 discloses a universal joint arrangement for damping vibration between drive and driven shafts, but it will not permit the driven shaft to maintain its preset rotational speed while the driver shaft speed varies.
While the structural arrangements of the prior art at first appearance have similarities with the structure of the invention, they differ in material aspects, primarily in that they do not permit the driven shaft speed to be maintained constant while the drive shaft speed is varied. This function is essential for the satisfactory operation of a high performance belt type printer. Moreover, a structure such as that of the invention permits other advantages that are not available with the prior art structures.